Since the electric motor was invented, centuries of development and progress thereof have brought about a lot of conveniences for human's daily life. From the original large-sized electric motor for the industrial purpose, the electric motor has now minimized to be used in the commodities or machine tools that might be used every day.
Machine tools commonly used in the industry, such as grinding machines, are mainly driven by the electric motor, as shown in R.O.C. patent Nos. M261316 and M288839. The machine tools disclosed in the above patents both have a main body including a motor and a tool shaft driven by the motor, wherein the tool shaft is equipped with two counter balancing masses to cause an eccentric rotation thereof, and the two counter balancing masses are linked to an actuation element, so that after the motor is switched on, the tool shaft can be driven to have an eccentric rotation and the two linked counter balancing masses can drive the actuation element to operate, thereby achieving the purpose of processing the workpiece. However, during the operation, it is easy that the heat produced by the motor is accumulated inside the machine tool, which not only might overheat the motor to reduce the operation time of the machine tool, but also might influence the durability of the machine tool.
An improvement as shown in China patent No. 2887526Y provided that the main body of the machine tool is formed to have an inlet and an outlet corresponding to the position of the motor, so that the external air can be introduced therein through the inlet and the heat inside can be discharged thereout through the outlet as the motor is operating. Thus, the heat produced by the motor can be dissipated and the problem of heat accumulation also can be solved. However, since the two counter balancing masses and the actuation element also will generate a large amount of heat during operation, and obviously, the structure described in China patent No. 2887526 only can dissipate the heat from the motor, the heat generated by the two counter balancing masses cannot be dissipated by the introduced air. Therefore, the durability of this type of machine tool is still deficient.
In view of above, US publication No. 2009/0239453 disclosed that a heat sink is sleeved around the two lower counter balancing masses, and thus the heat sink can operate synchronously with the two counter balancing masses for introducing air to dissipate the heat generated by the two counter balancing masses. However, since the blades of the heat sink are mounted around the lower counter balancing masses and at the same height therewith, when the machine tool is operating, the external air may initially enter the machine tool via the vent at the top handle, pass through the control circuit board mounted inside the handle and then turn downward to pass the motor, then be attracted by the heat sink, and finally pass through the counter balancing masses and be exhausted via an outlet. Therefore, practically, the air for passing through the two counter balancing masses has passed through the motor first and possesses a relatively higher temperature, so that the heat produced by the two counter balancing masses cannot be dissipated efficiently. Besides, the flowing path thereof is relatively long and not smooth, both of which might not only reduce the flow rate and flow amount, but also increase the dusts introduced with the external air to contaminate the circuit board and the motor.